5.2 Metal Connector and Fastener Materials and Fabrication

Most light gauge metal connectors are made from thin, flat sheets of steel that has been galvanized (has a zinc coating that was applied during fabrication). In flat sheet form, steel is sufficiently strong for connectors and is readily workable and relatively inexpensive, all of which make it well suited for connectors. Most fasteners (e.g., screws, bolts, nails) are also made of steel.

Unprotected (ungalvanized) steel is subject to corrosion, even in inland areas, and corrodes rapidly in salt air. To protect against corrosion, most light gauge metal connectors and fasteners are galvanized.

5.2.1 Galvanizing

Galvanizing is the process of coating steel with zinc.

In hot-dip galvanizing, the steel is carefully cleaned and then dipped into a vat of molten zinc. The high temperature melts the surface of the steel and forms several steel/zinc alloys that tightly bond the zinc coating to the steel base metal; various thicknesses of zinc coatings are achievable. The protective zinc coating still corrodes, but the corrosion of the zinc protects the steel base metal. Galvanized steel can degrade up to more than 50 times slower than ungalvanized steel in the same coastal environment.

Zinc can also be applied mechanically. The method typically involves tumbling the metal parts in acid and copper to mechanically weld a zinc coating onto a steel surface. This method is preferred for applying zinc to threaded parts, such as machine screws, because it creates a uniform thickness of the coating. As with hotdip galvanizing, various thicknesses of zinc coatings are achievable.

Mechanically galvanized fasteners are appropriate for some applications, but mechanically applied galvanized coatings are typically thinner and more brittle than hotdip galvanized coatings, so mechanically galvanized fasteners are discouraged for exterior applications, particularly in coastal environments. The IBC and IRC prohibit mechanical galvanizing for driven fasteners such as nails and timber rivets because mechanically galvanized coatings may deteriorate during installation. For these applications, hot-dip galvanized coated steel or stainless steel fasteners are recommended.

5.2.2 Protective Properties of Zinc

Galvanizing is particularly effective for steel because, unlike most other coatings, zinc sacrificially protects bare steel edges and scratches. The zinc surface near a scratch corrodes slightly faster than the zinc surrounding it and fills small scratches with zinc corrosion products, preventing the steel from rusting until the nearby zinc is consumed. The protection of bare edges and scratches offered by galvanizing is important because many connectors are fabricated after the steel sheet metal has been galvanized, and fabrication can remove the zinc and expose the base metal.

Zinc also differs from other coatings (or paints) and most metals by corroding at a relatively steady rate in most atmospheric exposures. Therefore, doubling the thickness of the zinc coating approximately doubles the protection period.

5.2.3 Standards and Codes

ASTM International (ASTM) has established national standards for galvanizing that are referenced by the I Codes, ASCE 24, and most local building codes. The standards are contained in the following:

• Hot-dip galvanized steel sheets used in the manufacture of metal connectors are covered in ASTM A653, Standard Specification for Steel Sheet, Zinc-Coated (Galvanized) or Zinc-Iron Alloy-Coated (Galvannealed) by the Hot-Dip Process (2015a).
• Metal connectors that are hot-dip galvanized after fabrication are covered in ASTM A153/A153M, Standard Specification for Zinc Coating (Hot-Dip) on Iron and Steel Hardware (2016).
• Hot-dip galvanizing of most fasteners is covered by ASTM A123, Standard Specification for Zinc (Hot-Dip Galvanized) Coatings on Iron and Steel Products (2015b).
• Mechanically applied galvanizing is covered in ASTM B695-04, Standard Specification for Coatings of Zinc Mechanically Deposited on Iron and Steel (2004), which states that Class 55 is the minimum thickness allowed to be in contact with preservative-treated lumber.

The I-Codes require that treated wood or the connector manufacturer’s recommendations be followed when connectors are in contact with preservative-treated lumber. In the absence of direction, the I-Codes require that a minimum G185 zinc-coated steel be used (some exceptions are provided). Use of Type 304 or Type 316 stainless steel is regarded as exceeding the minimum G185 zinc-coated steel code requirement for corrosion protection. Stainless steel types are explained in Section 8.2 of this Course.

5.2.4 Proprietary Coatings

For some types of fasteners, particularly screws, manufacturers use a proprietary coating rather than simple galvanization. One advantage of using a proprietary coating is that the coating can be more uniform to allow the threads to function normally. Manufacturers may have a building code evaluation report to show that their coating is equivalent to one of the prescriptive corrosion-resistant coatings required by the codes.